1. Field of the Invention
This invention relates to a method for recording an optical recording disc of phase change type.
2. Prior Art
Highlight is recently focused on optical recording media capable of recording information at a high density and erasing the recorded information for overwriting. One typical overwritable optical recording media is a phase change optical recording medium wherein a laser beam is directed to the recording layer to change its crystalline states whereupon a change of reflectance by the crystallographic change is detected for reproduction of the information.
The phase change optical recording media are of great interest since the medium can be overwritten by modulating the intensity of a single laser beam and the optical system of the drive unit used for their operation is simple.
When information is recorded in the optical recording medium of phase change type, the laser beam applied is of the power (recording power) such that the recording layer is heated to a temperature higher than the melting point. In the region where the recording power is applied, the recording layer is melted and thereafter quenched to form an amorphous record mark. When the record mark is erased, a laser beam of the power (erasing power) such that the recording layer is heated to a temperature higher than the crystallizing temperature and lower than the melting temperature is applied. The record mark to which the laser beam of erasing power is applied is heated to a temperature higher than the crystallizing temperature and then allowed to slowly cool to recover the crystalline state. Accordingly, in the optical recording media of the phase change type, the medium can be overwritten by modulating the intensity of a single laser beam.
In an optical recording medium of phase change type, thermal behavior in the recording and the erasure is greatly affected by the relative velocity of the laser beam in relation to the medium, namely, by the linear velocity of the medium. When the composition of the recording layer and the thermal structure of the medium are optimized to realize optimal properties at a particular linear velocity, the medium suffers from the problems as described below. When the linear velocity adopted is faster than the above-mentioned particular linear velocity, cooling rate of the recording layer will be too fast to render the crystallization of the record marks, namely, erasure difficult. On the other the hand, when the linear velocity adopted is slower than the above-mentioned particular linear velocity, cooling rate of the recording layer will be too slow, and amorphousizing of the record marks will be difficult. Formation of the record marks of accurate shape will then become difficult.
An optical recording medium of phase change type, however, is required to show satisfactory overwriting properties in a wide range of linear velocity. For example, a high linear velocity is required in order to reduce the time of overwriting while a real time recording is sometimes required in the recording of image or music. When a drive system compliant with a higher linear velocity is introduced in the market, it is usual that an economical drive system compliant only with the low linear velocity is simultaneously sold, and compatibility between the systems should be taken into consideration.
Optical recording methods attempting to support different linear velocities are disclosed, for example, in JP-A 7176/1997 and 138947/1997. These methods attempt to realize an optical recording medium of phase change type wherein the recording waveform for forming one record mark is constituted from a pulse train, and different linear velocities are supported by controlling width of each pulse and width of the downward pulse subsequent to each pulse depending on the linear velocity.
The inventors of the present invention, however, have found that an optical recording medium can not be made compliant with a wide range of linear velocity even if the recording waveform used were a pulse train and the width of each pulse and the subsequent downward pulse were controlled as described in JP-A 7176/1997 and 138947/1997.
In view of the situation as described above, an object of the present invention is to enable overwriting of an optical recording medium of phase change type whereby favorable properties are realized in a wide range of linear velocity.
In view of the situation as described above, an object of the present invention is to enable overwriting of an optical recording medium of phase change type whereby favorable properties are realized in a wide range of linear velocity.
The object as described above is attained by any one of (1) and (2) as described below.
(1) A method for recording an optical recording medium comprising a substrate and a recording layer of phase change type on the substrate; wherein
a pulse train is used as the recording waveform for laser beam modulation during formation of one recording mark, said pulse train including at least one unit pulse comprising an upward pulse and a subsequent downward pulse; and
relation (I):
0.25xe2x89xa6(TclH/TclL)xc3x97nxe2x89xa61.25xe2x80x83xe2x80x83(I).
is satisfied when width of the last downward pulse standardized by the reciprocal of linear velocity is TclH at a linear velocity of V and TclL at a linear velocity of V/n (n is a real number greater than 1).
(2) An optical recording method according to the above (1) wherein unit pulses excluding the top and the last unit pulses satisfy the relation (II):
0.55xe2x89xa6(TmpH/TmpL)/nxe2x89xa61.20xe2x80x83xe2x80x83(II)
when proportion of width of the upward pulses is TmpH at a linear velocity of V, and TmpL at a linear velocity of V/n.